This invention relates generally to an electromagnetic flowmeter system whose primary is excited by direct current or a current having a frequency lower than that of a standard commercial power source, and more particularly to a system in which a reference voltage proportional to the excitation current is detected in order to render the system insensitive to fluctuations in this current.
In recent years, magnetic flowmeters have been made available which make use of a low-frequency excitation wave in sinusoidal or rectangular form, such as one whose excitation frequency is well below that of the commercial power line frequency. One can, by means of a low-frequency excitation type magnetic flowmeter obtain a flow rate signal having an excellent signal-to-noise ratio; for unwanted magnetic coupling and/or electrostatic coupling between the excitation coil and the signal lead wires connected to the flow tube electrodes may thereby be reduced considerably.
A low-frequency excitation type magnetic flowmeter also requires an arrangement for eliminating fluctuations in the flow rate signal resulting from fluctuations in the excitation current for the electromagnet.
In order to eliminate unwanted fluctuations from the flow rate signal yielded by a magnetic flowmeter, the general practice is to provide a reference-voltage detecting circuit. This circuit is arranged to produce a reference-voltage proportional to the excitation current, the ratio of this reference-voltage to the flow rate signal being determined by means of a divider.
The reference-voltage is usually derived from the secondary winding of a reference-voltage detecting transformer whose primary winding is interposed in series with the excitation circuit of the electromagnet. This is done in order to isolate the divider from the excitation current. An isolation transformer arrangement for a reference voltage detection circuit in an electromagnetic flowmeter is disclosed in the Suzuki et al. U.S. Pat. No. 4,117,721 of Oct. 3, 1978.
In some prior art types of electromagnetic flowmeter systems in which the excitation-current frequency is well below 50 Hz or 60 Hz, the current transformer for deriving the reference voltage is very bulky as compared to those reqired when using a commercial power line frequency of 50 or 60 Hz for excitation. Without a bulky transformer for this purpose, one cannot obtain a reference voltage which is accurately proportional to the excitation current.
In one known form of a low-frequency excitation type electromagnetic flowmeter, the commercial power line source is isolated by a power transformer whose output voltage is applied to the excitation coil through an excitation-current detecting resistor to produce the reference voltage, the resistor thereby isolating the reference voltage from the commercial power line. The problem with this arrangement is that a power transformer of large capacity is required to handle a large excitation current, and the amount of heat radiated from this transformer is substantial. These factors militate against the reduction of the size and weight of the electromagnetic flowmeter.